Here's a summary:
The speaker compares their beloved to a summer's day, but finds them even more beautiful and lasting.
* The poem begins by comparing the beloved to a summer's day, acknowledging its beauty and pleasantness.
* However, the speaker quickly points out that summer days are often too short, too hot, or too stormy.
* The beloved, on the other hand, is "more lovely and more temperate," meaning they are consistently beautiful and unchanging.
* Summer's beauty fades, but the beloved's beauty will last "as long as men can breathe or eyes can see," suggesting a timeless and eternal beauty.
* The poem ends by stating that the beloved's beauty will never die, but will live on in poetry and remain forever young.
Overall, the poem celebrates the enduring beauty and lasting love the speaker feels for their beloved, making it a timeless expression of romantic devotion.